Nak Cheon Jeong, Omar K. Farha, Joseph T. Hupp*

Transcription

1 Supporting Information for A Convenient Route to High Area, Nanoparticulate TiO 2 Photoelectrodes Suitable for High-efficiency Energy Conversion in Dye-sensitized Solar Cells Nak Cheon Jeong, Omar K. Farha, Joseph T. Hupp* Department of Chemistry, Argon-Northwestern Solar Energy Research Center (ANSER), Northwestern University, 2145 Sheridan Road, Evanston, IL RECEIVED DATE (j-hupp@northwestern.edu) S-1. Materials and Methods. Materials. Titanium (IV) tetrachloride (90 mm in 20% of HCl solution, Aldrich), titanium (IV) isopropoxide [TIP, 97%, Aldrich], tetrabutylammonium hydroxide (TBAOH, 40% in water, Aldrich), tetramethylammonium hydroxide (TMAOH, 25% in water, Aldrich), hydroxypropyl cellulose (HPC, Aldirch), ethanol (EtOH, 96%, Mallinckrodt), isopropyl alcohol (IPA, 99.5%, Mallinckrodt), acetonitrile (99%, Mallinckrodt), commercial nanocrystalline TiO 2 paste (18-nm average-size TiO 2 NPs, DSL 18NR-T, Dyesol), TiO 2 paste for scattering layers (400-nm average-size TiO 2 NPs, WER4-0, Dyesol), N719 dye [(Bu 4 N) 2 {Ru(dcbpyH) 2 (NCS) 2 }, Dyesol], and Alconox detergent were used as received. Fluorine-doped tin-oxide glasses (FTO, 10 Ωcm -2 for anodes and 15 Ωcm -2 for cathodes) were purchased from Hartford glass for the electrodes. Chloroplatinic acid hexahydrate (H 2 PtCl 6 6H 2 O, ACR grade, Aldrich) was used for cathode. Iodine (I 2, 99.8%, Aldrich), guanidine thiocyanate (99%, Aldrich), 4-tert-butylpyridine (99%, Aldrich), valeronitrile (99.5%, Aldrich), and 1-butyl-3-methylimidazolium iodide (BMI + I -, 98%, TCI) for preparation of electrolyte solutions were used as received. Cells were assembled using thermoplastic Surlyn-1702 film (thickness = 60 µm, Dupont), indium coated copper wire (diameter = 0.25 mm, Arcor), conductive silver epoxy (type A and B, Chemtronix), and micro cover glasses (18 18 mm 2, VWR). Synthesis of TiO 2 NPs. The starting materials for synthesizing TBA-TiO 2 NPs were TIP, TBAOH, IPA, and H 2 O. First, 20 g of TIP was combined 30 g of IPA in a flask, with 4.6 g of TBAOH being combined with 76 ml of H 2 O in another flask. The TIP/IPA solution was the slowly added to the TBAOH/H 2 O solution with stirring at room temperature. Subsequently, the mixed solution was boiled with continuous stirring for 2 h. The initially turbid, white colloidal suspension became translucent and bluish-white suspension. When the volume of the colloidal solution had decreased to ca. 40 ml (due to boling), an additional 40 ml of H 2 O was added. The mixture was further boiled until the volume was reached ca. 45 ml. After the transparent colloidal TiO 2 suspension had cooled to room temperature, it was transferred to a Teflon-lined autoclave and allowed to react at 150 C for 2.5 h. At this point, the SI-1

2 concentration of TBA-TiO 2 NPs was 6.5 wt%. TMA-TiO 2 NPs where prepared in the same way, except with TMAOH in place of TBAOH. Preparation of α-terpineol-based TiO 2 Paste. A 10 wt% solution of HPC in EtOH was prepared. 1.4 g of α-terpineol was then added to 1.0 g of the HPC-containing solution in a polypropylene bottle. 3.1 g of TBA-TiO 2 colloidal suspension was diluted with 9 g of EtOH in a vial. The diluted suspension was then added to the solution of HPC and α-terpineol in drop-wise fashion. The combined mixture was then stirred with a magnetic bar until a highly viscous paste formed. Typically about 14 h of stirring was required. Preparation of TiO 2 films. FTO glass plates (10 Ω cm -2 ) where cut into 15 mm x 15 mm squares. Contaminants were removed from the squares by sonicating them in a 10% aqueous solution of Alconox detergent for 30 min. After washing with copious amount of distilled water, the FTO were refluxed in a 40 mm solution of TiCl 4 in IPA solution for 20 min and then removed from the solution and placed in an oven at 420 C for 30 min. For the fabrication of porous TiO 2 films, either the TBA- TiO 2 paste or the commercial TiO 2 paste was deposited on FTO squares by the doctor-blade method, using parallel double-layers of Scotch Magic tape as a mask (Com-TiO 2 films) or triple layers (TBA- TiO 2 films). (Attempts to prepare thicker films by using a larger number of spacer layers were unsuccessful, due to film cracking.) After drying the films for 30 min at 110 C, the tape was removed. Films were then calcined at 400 C for 30 min. For films used to for UV-Vis spectral measurements (with or without dye), no additional processing was done. For films destined for photovoltaic studies, a TiO 2 scattering layer was deposited on top of the above TiO 2 layer using ~400 nm size TiO 2 particles prior to the above-described calcination step. After calcination, the dimensions of each TiO 2 film were reduced to ~3 3 mm 2 using a razor blade. Finally, the TiO 2 coated FTO glasses were again refluxed in a 40 mm solution of TiCl 4 in IPA and calcined at 370 C for 15 min. Dye adsorption was accomplished by soaking the TiO 2 films in a 0.5 mm solution of N719 in 1:1 of acetonitrile:etoh as solvent. After 24h, the films were washed with copious amount of the above solvent and dried with a nitrogen stream prior to use. Preparation of Platinized Cathodes. FTO glass plates were cut into 20 mm x 20 mm squares. A 0.3 mm diameter hole was drilled in each glass. Contaminants on the FTO glasses were also removed using the above method. A 5 mm solution of H 2 PtCl 6 in EtOH solution was dropcast (1 drop; ca. 10 microliters) on each square and allowed to be dry in a capped polycarbonate Petri dish. Finally, the platinized squares were calcined at 380 C for 30 min. Assembly of Photovoltaic Cells. N719-adsorbed FTO glass and platinized FTO glass were sealed together by melting a 60-µm-thick Surlyn polymer film on a hotplate at 170 C. Indium-coated copper wires were connected on each electrode with silver epoxy. Then, the epoxy was dried at 110 C SI-2

3 for 40 min M 1-Butyl-3-methylimidazolium iodide, 0.03 M I 2, 0.10 M guanidine thiocyanate, and 0.50 M 4-tert-butylpyridine in 3 ml of the mixture of acetonitrile (85 Vol%) and valeronitrile (15 Vol%) were uses as electrolyte. ~30 µl of the electrolyte was dropped onto the drilled hole and then, the electrolyte was vacuum-loaded into the cell. After residual electrolyte on the hole was removed, the hole was sealed by melting a sheet of Surlyn polymer film that was inserted between the backside of the FTO and a micro cover glass slide. Finally, a photo-mask with an aperture was applied on top of the active area of each cell. For discussions, see: (1) G.-W. Lee, et al., Solar Energy 2010, 84, , and (2) S. Ito, et al., Prog. Photovoltaics 2006, 14, Instrumentation. Scanning electron microscopy (SEM) images were obtained from a FE-SEM (Hitachi S-4800) operated at an acceleration voltage of 10 kv, after samples were coated by Au-Pt alloys with the thickness of 3 nm. Transmission electron microscope (TEM) images were obtained from a JEOL JEM-2100F at an acceleration voltage of 200 kv. X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku diffractometer (XDS 2000) using the monochromatic beam of nickel-filtered Cu Kα. UV-Vis spectra of samples were recorded on a Varian Cary 5000 UV-VIS-NIR spectrophotometer. The photocurrent-density-to-applied-voltage (J-V) curves and IPCE curves of photovoltaic cells were obtained using a home-made set-up which consists of xenon lamp, an AM 1.5 light filter, and a CHI 1202 Electrochemical Analyzer (CHI instruments). The power of filtered light was calibrated by optical power meter (OPM) to 100 mwcm -2. SI-3

4 S-2 A B C Figure SI-1. Changes of appearance of TiO 2 colloidal suspension over time, under hydrothermal reaction conditions. Photographs were taken after (A) 5 min, (B) 60 min, and (C) 80 min. The photographs show that the suspension becomes progressively more transparent. SI-4

5 S-3 A 50 nm B C Figure SI-2. TEM images of (A) TMA-TiO2, (B) TBA-TiO2, and (C) Com-TiO2 NPs. While the ComTiO2 NPs range in the size from ~10 to ~35 nm, the TMA-TiO2 and TBA-TiO2 NPs range in size from ~5 to ~20 nm. SI-5

6 S-4. Determination of the sizes of crystalline anatase TMA-, TBA-, and Com-TiO 2 NPs The average sizes of them were determined by using Debye-Scherrer formula below. Kλ Crystal size = P cosθ where, K is the shape factor of crystallite, λ is wavelength of X-ray (1.54 Å for Cu Kα), P is the peak width (full width at half maximum, fwhm) of a peak, and θ is the diffraction angle. For all three materials, the observed peaks are characteristic of the anatase form of titanium dioxide. For the calculation of average sizes, we chose the (200) diffraction peak, whose width for TMA-, TBA-, and Com-TiO 2 NPs was 0.97, 0.98, and 0.51, respectively. From these results, the calculated average sizes of TMA-, TBA-, and Com-TiO 2 NPs were 9.0, 8.9, and 17 nm, respectively. (101) (004) (200) (105) (211) Com-TiO 2 TBA-TiO 2 TMA-TiO θ Figure SI-3. XRD patterns of anatase crystalline TMA-TiO 2, TBA-TiO 2, and Com-TiO 2 NPs as indicated. The peak widths of (200) diffraction plane (2θ = 48.02) of TMA-TiO 2, TBA-TiO 2, and Com- TiO 2 NPs are 0.97, 0.98 and 0.51, respectively. SI-6

7 S-5 Figure SI-4. Photograph of TBA-TiO 2 paste. SI-7

8 S-6 A 8.1 µm 5 µm B 11.0 µm 5 µm Figure SI-5. Cross-sectional SEM images of (A) TBA-TiO 2 film and (B) Com-TiO 2 film. SI-8

9 S-7 dv/dr (cm 3 g -1 Å -1 ) X TBA-TiO 2 Commercial TiO Pore diameter (nm) Figure SI-6. Pore-size distributions of films fabricated with TBA-TiO 2 (red) and Com-TiO 2 (blue). SI-9

10 S-8. BJH surface area of TBA-TiO 2 and commercial TiO 2 films per unit volume. The BJH desorption pore volume per a gram (V P/W ) and the BJH surface area per a gram (A W ) were measured, and then, the surface area per unit volume (A V ) was calculated by using the above values with the simple equation below. A A = = ( Q V 1/ ρ ) / W / V TiO2 / W TiO2 VP / W + VTiO 2 / W where V TiO2/W is the volume of TiO 2 NPs per a gram and ρ TiO2 is the density of TiO 2. Table SI-1. Values for calculating surface area per unit volume ( A V ) in eq. S1. TBA-TiO 2 film Com-TiO 2 film ρ TiO2 (gml -1 ) V TiO2/W (mlg -1 ) V P/W (mlg -1 ) A /W (m 2 g -1 ) A /V (m 2 ml -1 ) SI-10

11 S-9 A. TBA-TiO 2 (8 µm-thick) B. Commercial TiO 2 (11 µm-thick) Figure SI-7. Photographic images of a N719 dye-loaded TBA-TiO 2 film (A) and a Com-TiO 2 film (B). SI-11

12 S Absorbance TBA-TiO 2 film Com-TiO 2 film Wavelength (nm) Figure SI-8. Thickness-normalized UV-Vis absorption spectra of N719-coated TBA-TiO 2 and Com- TiO 2 films. SI-12

13 S-11 J (ma cm -2 ) TBA-TiO Com-TiO Light intensity (mw cm -2 ) Figure SI-9. Photocurrent densities as a function of intensity of AM 1.5 light for DSSCs containing TBA-TiO 2 or Com-TiO 2 based photoelectrodes. SI-13